Slips are used for various downhole tools, such as bridge plugs and packers. The slips can have inserts or buttons to grip the inner wall of a casing or tubular. Inserts for slips are typically made from cast or forged metal, which is then machined and heat-treated to the proper engineering specifications according to conventional practices.
Inserts for slips on metallic and non-metallic tools (e.g., packers, plugs, etc.) must be able to engage with the casing to stop the tools from moving during its operation. On non-metallic tools, such as composite plugs, the inserts can cause the non-metallic slips to fail when increased loads are applied. Of course, when the slip fails, it disengages from the casing. On non-metallic tools, the inserts also need to be easily milled up to assist in the removal of the tools from the wellbore.
When conventional inserts are used in non-metallic slips, they are arranged and oriented as shown in FIG. 1A, for example. The slip 20 is disposed adjacent a mandrel 10 of a downhole tool, such as a bridge plug, a packer, or the like. As shown in FIG. 1B, the slip 20 moves away from the mandrel 10 and engages against a surrounding tubular or casing wall when the slip 20 and a cone 12 are moved toward one another. Either the slip 20 is pushed against the ramped surface of the cone 12, the cone 12 is pushed under the slip 20, or both.
FIG. 2A illustrates a side cross-section of a slip 20 having holes 23 according to the prior art for inserts (not shown), and FIG. 2B illustrates a side cross-section of the slip 20 with inserts 30 disposed in the holes 23. FIG. 2C illustrates a front view of the slip 20 with the holes 23 for the inserts (not shown). The slip 20 can have a semi-cylindrical shape. The holes 23 in the surface of the slip 20 can be an array of blind pockets. The inserts 30 are anchor studs that load into the holes 23 and can be held with a press fit or adhesive.
Examples of downhole tools with slips and inserts such as those above are disclosed in U.S. Pat. Nos. 5,984,007; 6,976,534; and 8,047,279. Other examples include Halliburton Obsidian® and Fas Drill® Fusion composite plugs and Boss Hog frac plugs. (OBSIDIAN and FAS DRILL are registered trademarks of Halliburton Energy Services, Inc.)
One particular type of downhole tool having slips is a composite fracture plug used in perforation and fracture operations. During the operations, the composite plugs need to be drilled up in as short of a period of time as possible and with no drill up issues. Conventional composite plugs use metallic wicker style slips, which are composed of cast iron. These metallic slips increase the metallic content of the plug and can cause issues during drill up in horizontal wells, especially when coil tubing is used during the milling operation.
Due to the drawbacks of cast iron slips, composite slips having inserts, such as described above, are preferably used to reduce the issues associated with metallic slips. Unfortunately, a large amount of metallic debris can still collect at the heel of the well and cause drill up problems when composite slips having inserts are used on tools. When composite slips are used, for example, the inserts are typically composed of carbide, which is a dense and heavy material. In other developments, it is known to use a composite slip with two different insert materials (i.e., ceramic and metallic) in the same insert, such as described in U.S. Pat. No. 6,976,534.
In any event, when the downhole tool having slips with carbide inserts are milled out of the casing, the inserts tend to collect in the casing and are hard to float back to the surface. In fact, in horizontal wells, the carbide inserts may tend to collect at the heel of the horizontal section and cause potential problems for operations. Given that a well may have upwards of forty or fifty bridge plugs used during operations that are later milled out, a considerable number of carbide inserts may be left in the casing and difficult to remove from downhole.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.